The present invention generally relates to conveying or feeding sheets or sheet material such as, for example, corrugated blanks, in a box finishing machine. In such machine it is important that the sheets are fed in synchronism with the operations performed at the stations along the machine, such operations being, for example, printing, slotting and scoring, folding and gluing. In the box finishing machine art, synchronous feeding of the sheets relative to the cycle of operation at the various stations along the machine is often referred to as "register feeding" or "feeding in register". In order for the operations such as printing, slotting, scoring, folding and gluing to be performed at the right locations on the sheet, it is obvious that the sheet must arrive at the stations at precisely the right times.
In a box finishing machine, for example, corrugated blanks are fed from a vertical hopper one by one from beneath the hopper by means of a first feeder which sequentially transports the blanks from the bottom of the hopper to a second feeder positioned at the beginning or inlet of the box finishing machine. In conventional machines, the second feeder may be nip rolls or feed rolls. In the corrugated box art, the second feeder could be termed a transfer conveyor, pull conveyor or feeder conveyor.
Feed rolls or nip rolls include an underlying roll typically having a knurled steel surface and an upper roll having for example a steel core and a grooved rubber surface layer. The sheet or corrugated blank being fed is of course gripped between the rolls and fed along the path of the finishing machine. The area of contact with the corrugated blank is limited to that which occurs at the nip of the feed rolls. Consequently, it is necessary to provide sufficient force at the nip to ensure proper gripping of the corrugated blank. The result is that the blank being fed is susceptible to crushing or deformation, and furthermore it will not be gripped with sufficient force if the gap between the rollers is not set to precise dimension. Moreover, the precise setting of the gap is not predictable with such rolls. In addition, the deformation of the flexible or deformable feed roll surfaces causes variation in surface speed resulting in loss of register and roll wear.
More recently a vacuum type conveyor has been used in which for example a wheel or belt conveyor is contained in a vacuum box so that the vacuum holds the sheet or blank on the belt or wheels of the conveyor. However, the problem with this method is that if the vacuum in the vacuum box is constant, large air losses occur in the spaces between successive sheets or blanks being fed thus requiring a very large volume of air movement and vacuum source, not to mention the noise and power requirements that attend such installations.
In an attempt to overcome this problem, application of the vacuum is timed with the flow of the sheets or blanks. However this imposes a limitation on the speed of the feeding process and in turn production while further requiring complicated and expensive mechanisms in order to effect the periodic application of vacuum in timed relationship with the flow of sheets or blanks. In addition, with a vacuum system, the amount of vacuum that can be applied to the sheets is limited and thus loss of register can result.
Another attempt to improve feeding in this art is disclosed in my U.S. Pat. No. 5,183,251. While the conveyor disclosed there has advantages over nip rolls and vacuum conveyor, it involves the handling of positive air flow to hold the blank on the conveyor belt. The flow of air can result in problems with dust in downstream operation of printing.